1. Technical field of the Invention
This invention relates to an improved adsorbent useful in selective adsorption of unsaturated hydrocarbons and the manufacture of the adsorbent. More particularly, the invention relates to an adsorbent having a high degree of selectivity and affinity for olefin molecules and also having high adsorption capacity for olefins, and a process for producing the same. More specifically, this invention relates to a process for separating ethylene and /or propylene employing the novel, specially prepared adsorbent, from a mixed gas containing ethylene and/or propylene together with a component selected from the group consisting of H.sub.2, N.sub.2, Ar, He, CH.sub.4, C.sub.2 H.sub.6, C.sub.3 H.sub.8, CO.sub.2, CO and mixtures thereof.
2. Prior art
Unsaturated hydrocarbons such as ethylene and propylene are basic raw materials in synthetic chemistry. Demand for these gases is ever increasing. These are produced by naphtha/natural gas cracking or by dehydrogenation of paraffin. Invariably these are obtained as mixtures necessitating separation before their use. Prior art processes conventionally employed for the separation of ethylene from ethane and propylene from propane involved low temperature and/or high-pressure distillation. These processes are highly energy intensive and difficult to achieve and therefore, are not commercially very attractive. A common conventional process for the separation of mixture of ethane-ethylene is carried out at -25.degree. C. and 320 psig in a distillation column containing over 160 trays and propane-propylene at -30.degree. C. and 30 psig pressure in a distillation column containing over 200 trays [Keller, G. E; Marcinkowsky, A. E; Verma, S. K; Williamson, K. D.; Olefin Recovery and Purification via silver complexation; Li, N. N.; Calo, J. M.; In Separation and Purification Technology, Eds , Marcel Dekker, New York, 1992)]. It is now generally accepted that separations of ethane-ethylene and propane-propylene by distillation are some of the largest energy consuming separation processes in the petrochemical industry. Therefore, there is a large demand of an efficient and low cost, low energy process for the separation of olefins, particularly, ethane and propane from gas mixtures containing them, especially in view of the fact that demand for ethylene and propylene is ever increasing. For instance, world wide ethylene capacity of 80.8 million metric tons (mmt) is expected to grow to 122.1 mmt by 2005.
For the past several years, various attempts have been made to develop alternative technologies such as adsorption, chemical absorption and membrane separation processes. Of the various alternate technologies, adsorption appears to be the most promising [Eldridge, R. B., Olefin/Paraffin Separation Technology: A Review., Ind. Eng.Chem. Res., 32, 2208, 1993]. Conventional adsorbents such as activated alumina, activated carbon, silica gel and zeolites do not show good selectivity for olefins over paraffins. Hence, development of a suitable adsorbent has become a key factor for the successful development of adsorption process.
Some of the adsorbents that have been reported for paraffin-olefin separation are CuCI [Gilliland, E. R., Bliss, H/ L., Kip, C. E., Reaction of olefins with solid cuprous halide, J Am. Chem. Soc., 63, 2088, 1941; Gilliland, E. R., Concentration of Olefins, U.S. Pat. No. 2,369, 559, 1945, Long, R. B. Separation of Unsaturates by Complexing with solid copper salts., In Recent Development in Separation Science; Li, N.M. Ed., CRC Press, Cleveland, 1972) ion exchanged zeolites [Rosback, D. H., Olefin Separation Process Using Copper-Exchanged Type X Zeolites., U.S. Pat. No. 3, 755, 153, 1973; Rosback, D. H., Adsorbing Olefins with a Copper-Exchanged Type X Zeolite., U.S. Pat. No. 3, 720, 604, 1973); polymer supported aluminium silver chloride [Hirai, H., Kurima, K., Wada, K., Komiyama, M., Selective Ethylene Adsorbents Composed of Copper (I) Chloride and Polystyrene Resins having Amino Groups., Chem Lett., 1513, 1985; Hira, H., Hara, S., Komiyama, M., Polystyrenc-Supported Aluminium Silver Chloride as selective Ethylene Adsorbent., Angew. Makromol. Chem., 130, 207, 1985; Hirai, H., Polymer Complex for the Separation of Carbon Monoxide and Ethylene., In Polymers For Gas Separation; Toshima, N. Ed., VCH Publishers, Inc., New York, Chapter 7, 1992); and copper containing resins (Dielacher, M.; Hansen, U. Separation of Unsaturated Compounds from Liquid Hydrocarbon Mixtures Containing the Same, U.S. Pat. No. 3,979,280, 1976). Most of these prior art adsorbcnts, particularly, ion exchanged zeolites, polymer supported silver chloride and copper containing resins suffer from one or the other drawbacks such as slow adsorption kinetics, poor adsorption capacity, and/or selectivity. More recently, Yang and Kikkinides (New Sorbents for Olefins/Paraffin Separations by Adsorption via O-complcxation. AIChE J 41, 509, 1995) and Cho and coworkers [Wu, Z., Han, S.S., Cho, S.H., Kim, J. N., Chue, K. T., Yang, R.T., Modification of Resin-Type Adsorbents for Ethane/Ethylene Separation, Ind Eng. Chem. Res., 36, 2749, 1997] have reported more promising adsorbents. Among the adsorbents reported by them, Ag.sup.+ resin and CuCI/Al.sub.2 O.sub.3 showed high olefin adsorption capacity and good selectivity. However, ethylene and propylene sorption kinetics on Ag.sup.+ resin are slow. Xie et al (Ind. Eng. Chem. Res., 36, 27749, 1997) have reported more promising adsorbants. CuCI/Al.sub.2 O.sub.3 is CuCI dispersed on .gamma.-Al.sub.2 O.sub.3 by monolayer dispersion technique and hence, is obtained in powder form. For commercial use this adsorbent needs to be formed into pellets, which leads to reduction in adsorption capacity and selectivity. Further, adsorbent formulations prepared using Cu[I] compounds arc unstable and easily get oxidised to Cu(II) leading to a loss in adsorption capacity and selectivity of the adsorbent. Prior art also reports a series of adsorbents containing Cu(I). (Xie, Y. C., Tang Y. Q., Spontaneous Monolayer Dispersion of Oxides and Salts onto Surfaces Supports: Applications to Heterogenous Catalysis, Advances in Ccitalysis, 1, 37, 1990; Xie, Y. C., Bu, N.Y., Liu, J. Yang, G., Qiu, J. G., Yang, N. F., Tang,. Y. C., Adsorbents for Use in the Separation of Carbon Monoxide and/or Unsaturated Hydrocarbons from Mixed Gases. U.S. Pat. No. 4,917, 711, 1990) These were also prepared in powder form. Hence, these adsorbents also suffer from the above-mentioned drawbacks. Recently, a selective adsorbent with AgNO.sub.3 dispersed on activated alumina with superior adsorption capacity, selectivity and rates of adsorption has been invented (Korean Patent Application No. 24912 filed on Jun. 26, 1998 and U.S. Patent Application No. 09/209,431 dated Dec. 11, 1998 by the present applicants). This adsorbent while, producing excellent and most unexpected results as compared to the adsorbents taught by the prior art exhibited an ethylene to ethane adsorbtion ratio of approximately 6 to 7. Considering the tremendous commercial need for olefins particularly, ethylene and propylene, the inventors of the present application were constantly in a search for an adsorbent which could provide even higher ethylene to ethane and propylene to propane adsorbtion ratio than the adsorbent of the co-pending Korean and U.S. applications referred to above and which also has a much higher rate of adsorbtion as compared to the known adsorbents.